Hitherto, various structures for a photoelectric conversion element converting optical energy into electrical energy have been suggested. Among these, in a dye-sensitized solar cell (Graetzel type) suggested by Graetzel et al, Federal Institute of Technology, Lausanne, Switzerland, 1991, a conversion efficiency of the same level as amorphous silicon was achieved using relatively-cheap materials and simple and easy manufacturing processes. Accordingly, the Graetzel type dye-sensitized solar cell is expected to be put into practical use as a next-generation solar cell.
The Graetzel type dye-sensitized solar cell includes a semiconductor electrode in which a semiconductor layer absorbing dye having a light-absorbing function is formed on a conductive substrate, a counter electrode opposed to the electrode and formed of a conductive substrate, and an electrolyte layer (charge transport layer) disposed between both electrodes. In the Graetzel type dye-sensitized solar cell, electrons are injected into the semiconductor electrode from an excited dye having absorbed light and the electrons moves to the dye by an oxidation reaction of a redox agent in the electrolyte layer. The redox agent in the electrolyte is reduced again in the counter electrode, whereby the cell works. Such photochemical reaction cells using a dye sensitization effect were known in the past. However, in the Graetzel type dye-sensitized solar cell, the effective reaction surface area increases by 1000 times by using a porous titania electrode formed by sintering particulates as the semiconductor electrode, thereby extracting a larger photocurrent.
An example of the dye used in the Graetzel type dye-sensitized solar cell is a spectral sensitizing dye including a transition metal complex as described in Patent Document 1. More specifically, cisbis(isothiocyanato)-bis-(2,2′-bipyridil-4,4-dicarboxylate)ruthenium(II) bis-tetrabutylammonium complex (so-called N719) which is a kind of bipyridine complex and which has an excellent effect as the sensitizing dye is typically used as the spectral sensitizing dye. In addition, cisbis(isothiocyanato)-bis-(2,2′-bipyridil-4,4′-dicarboxylate) ruthenium(II)(so-called N3) which is a kind of bipyridine complex or tris(isothiocyanato)(2,2′:6′,2″-terpyridil4,4′,4″-tricarboxylate) ruthenium(II)tros-tetrabutylammonium complex (so-called black dye) which is a kind of tripyridine complex is typically used. In recent years, organic materials not containing a metal complex, such as derivatives of coumarin-based materials, have been reported.
Regarding the electrolyte layer used in the Graetzel type dye-sensitized solar cell, a method of interposing an electrolyte, which is obtained by dissolving a redox agent containing iodine and/or iodide ions in a nitrile-based organic solvent or an ionic liquid with low volatility, between the semiconductor electrode and the counter electrode is typically used. Since iodine with high corrosivity is used in such a method, Patent Document 2 discloses that the electrolyte is made into a pseudo-solid by gelation so as to prevent leakage of liquid to the outside. It was also reported that 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radical is used as a redox agent not containing iodine (Non-Patent Document 1).